function LimitDrag(id){
    Drag.call(this, id);
}
for(var i in Drag.prototype){
    LimitDrag.prototype[i] = Drag.prototype[i];
}
LimitDrag.prototype.fnMove=function(ev){
    var oEvent = ev || event;
    var l = oEvent.clientX - this.disX;
    var t = oEvent.clientY - this.disY;
    if(l < 0){
        l = 0;
    }else if(l > document.documentElement.clientWidth - this.oDiv.offsetWidth){
        l = document.documentElement.clientWidth - this.oDiv.offsetWidth;
    }
    this.oDiv.style.left = l + 'px';
    this.oDiv.style.top = t + 'px';
}